Thermoplastic resin composition containing titanium dioxide pigment, and process for preparing same

ABSTRACT

A resin composition comprising a thermoplastic resin and a titanium dioxide (TiO 2 ) pigment and having a moisture content of 10-1,400 ppm as determined by a specific method. A preferable resin composition is made by (i) atomizing 0.1-3.0 wt. parts of an organopolysiloxane within a fluidized powder layer of 100 wt. parts of TiO 2  pigment; (ii) mixing 1-5 wt. parts of a metallic soap with 100 wt. parts of the TiO 2  pigment; (iii) heat-treating the thus-obtained mixture at 70-200° C. for 10 min-2 hours; and then, (iv) mixing a thermoplastic resin with the heat-treated mixture. The resin composition is used for coating paper or film, when it contains 20-60 wt. % of TiO 2  pigment. The resin composition is used as a master batch for the preparation of a resin composition for coating paper or film, when it contains 30-80 wt. % of TiO 2  pigment.

This is a divisional of application Ser. No. 08/906,668 filed Aug. 8,1997, now U.S. Pat. No. 6,017,993 the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a thermoplastic resin composition containing atitanium dioxide pigment, and a process for preparing the thermoplasticresin composition.

The titanium dioxide pigment-containing thermoplastic resin compositionis suitable as a raw material for a highly white film and especially acoating material-for photographic resin-coated paper, and a master batchfor the preparation of a thermoplastic resin composition used as acoating material for coated paper.

(2) Description of the Related Art

To improve dispersibility of a titanium dioxide pigment in a resin, atechnique for coating a titanium dioxide pigment particle with aninorganic surface treating agent is generally employed. Namely, about0.1 to 2% by weight of an inorganic surface treating agent is used as acoating material which includes aluminum oxide hydrate, silicon oxidehydrate, titanium oxide hydrate, zirconium oxide hydrate, zinchydroxide, magnesium hydroxide, manganese compounds and phosphoric acidcompounds. Of these, aluminum oxide hydrate and silicon oxide hydrateare preferably used.

As means for coating one surface or both surfaces of a substrate such aspaper or a polyester film with a thermo-plastic resin compositioncontaining a titanium dioxide pigment, an extrusion lamination processis generally employed.

In recent years, a high degree of whiteness and a high degree of opacityare required for coated paper and film, and therefore a thermoplasticresin composition containing a titanium dioxide pigment at a highconcentration is used. However, the dispersion of titanium dioxidepigment in the conventional thermoplastic resin composition is poor, andthus, the state of a titanium dioxide coating is poor. Namely, thecoated paper and film have a poor surface smoothness or bubbles withinthe coating.

SUMMARY OF THE INVENTION

In view of the foregoing, a primary object of the present invention isto provide a thermoplastic resin composition containing a titaniumdioxide pigment, which is capable of forming a film or coatingexhibiting a satisfactory surface state on paper, even when theconcentration of titanium dioxide pigment in the thermo-plastic resincomposition is high.

Another object of the present invention is to provide a thermoplasticresin composition containing a titanium dioxide pigment, which is usedas a master batch for the preparation of a titanium dioxidepigment-containing thermo-plastic resin composition capable of formingthe above-mentioned coating or film.

Still another object of the present invention is to provide a process bywhich a titanium dioxide pigment-containing thermoplastic resincomposition can be advantageously prepared.

In one aspect of the present invention, there is provided athermoplastic resin composition comprising a thermoplastic resin and atitanium dioxide pigment; the moisture content in the resin compositionbeing in the range of 10 of 1,400 ppm as determined by a method wherein3 g of the thermoplastic resin composition is humidified in anatmosphere maintained at a temperature of 60° C. and a relative humidityof 60% for 4 hours, exposed to a stream of air having an absolutehumidity of 0.009 kgH₂ O/kg of dry air at a temperature of 80° C. for 4hours, and then the resin composition is deaerated at a temperature of300° C. for 30 minutes, and the moisture content is calculated by thefollowing equation:

    A=B/C

wherein A is moisture content, B is content (g) of moisture as measuredby the Karl Fischer method, and C is the content (g) of titanium dioxidepigment in the resin composition.

The content of a titanium dioxide pigment in the thermoplastic resincomposition is usually in the range of 20 to 80% by weight based on theweight of the resin composition. More specifically, when thethermoplastic resin composition is used for forming a coating on paperor a film, the content of a titanium dioxide pigment in thethermoplastic resin composition is usually in the range of 20 to 60% byweight, and, when the thermoplastic resin composition is used as amaster batch for the preparation of a thermoplastic resin compositionfor forming the coating, the content of a titanium dioxide pigment inthe thermoplastic resin composition is usually in the range of 30 to 80%by weight.

Preferably the titanium dioxide pigment to be incorporated in thethermoplastic resin is a particle of surface-treated titanium dioxidepigment which is prepared by treating a titanium dioxide particle with0.1 to 3.0 parts by weight of an organopolysiloxane and 1 to 5 parts byweight of a metallic soap, based on 100 parts by weight of the titaniumdioxide pigment.

In another aspect of the present invention, there is provided a processfor preparing a thermoplastic resin composition comprising athermoplastic resin and a titanium dioxide pigment, which comprises thesteps of:

atomizing 0.1 to 3.0 parts by weight of an organopolysiloxane asdroplets having an average diameter of 1 to 100 μm within a fluidizedpowder layer of 100 parts by weight of a titanium dioxide pigment;

mixing 1 to 5 parts by weight of a metallic soap with 100 parts byweight of the organopolysiloxane-treated titanium dioxide pigment;

heat-treating the thus-obtained mixture at a temperature of 70 to 200°C. for a period of 10 minutes to 2 hours; and then

mixing a thermoplastic resin with the heat-treated mixture.

The titanium dioxide pigment in the moisture content-controlledthermoplastic resin composition exhibits good dispersibility, and acoating formed on paper or film from the thermoplastic resin compositionhas a satisfactory surface smoothness and does not have bubbles therein.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Titanium dioxide used in the present invention may have any structure ofanatase-type and rutile-type. The process for preparing the titaniumdioxide is not particularly limited and may be prepared by, for example,a chlorine process and a sulfuric acid process. The titanium dioxidepreferably has an average particle diameter of 0.1 to 0.3 μm in view ofoptical characteristics of the thermo-plastic resin composition. With anaverage particle diameter smaller or larger than this range, opticalcharacteristics such as light scattering and shade are deteriorated.

To enhance light stability and dispersibility of titanium dioxide, thetitanium dioxide is preferably subjected to a surface treatment with aninorganic surface treating agent. The surface treating agent may beconventional and includes inorganic metal compounds such as aluminumoxide, silicon oxide and zirconium oxide, and hydrates thereof.

Of the inorganic surface-treating agents, aluminum oxide hydrate ispreferable. The amount of aluminum oxide hydrate used is preferably inthe range of 0.1 to 0.8 part by weight expressed in terms of aluminumoxide based on 100 parts by weight of titanium dioxide. If the amount ofaluminum oxide hydrate is smaller than this range, the catalytic actionof titanium dioxide due to light cannot be desirably controlled and theresin composition is apt to be deteriorated with the lapse of time.Further the dispersibility of titanium dioxide in the thermoplasticresin becomes poor. If the amount of aluminum oxide hydrate is largerthan this range, the amount of water accompanying aluminum oxideundesirably increases and the melt-extrusion lamination becomesdifficult.

An organopolysiloxane preferably used for coating titanium dioxide inthe present invention is a compound having a siloxane bond, representedby the following formula: ##STR1## wherein R₁, R₂, R₃, R₄, R₅ and R₆independently represent a hydrogen atom, a hydroxyl group, an alkylgroup having 1 to 7 carbon atoms, a cycloalkyl group having 3 to 8carbon atoms, an aryl group having 6 to 14 carbon atoms, an aralkylgroup having 7 to 12 carbon atoms, an alkenyl group having 2 to 8 carbonatoms, an alkoxy group having 1 to 7 carbon atoms, an aryloxy grouphaving 6 to 14 carbon atoms, an aralkyloxy group having 7 to 12 carbonatoms, an alkenyloxy group having having 2 to 8 carbon atoms, and n isan integer of at least 1.

As specific examples of R₁ through R₆, there can be mentioned an alkylgroup such as methyl, ethyl, propyl, butyl or hexyl, an aryl group suchas phenyl, naphtyl or anthryl, an aralkyl group such as benzyl,phenethyl or naphthylmethyl, an alkenyl group such as vinyl, allyl orstyryl, an alkoxy group such as methoxy or ethoxy, an aryloxy group suchas phenoxy, naphthyloxy or anthryloxy, an aralkyloxy group such asbenzyloxy, phenethyloxy and naphthylmethyloxy, an alkenyloxy group suchas vinyloxy, allyloxy or styryloxy. A glycidyl group may also beincluded.

As specific examples of the organopolysiloxane, there can be mentionedchain organopolysiloxanes such as methyl hydrogenpolysiloxane anddimethyl polysiloxane, cyclic dimethyl polysiloxane, methylmethoxypolysiloxane, ethylethoxy polysiloxane, methylethoxy polysiloxane,cyclic diphenyl polysiloxane and cyclic methylphenyl polysiloxane. Theseorganopolysiloxanes may be used either alone or in combination.

By treating titanium dioxide with the organopolysiloxane, activehydroxyl groups present-on the surface of titanium dioxide are reactedwith the organopolysiloxane, and a uniform and dense coating oforganopolysiloxane is formed. To enhance the efficiency of thisreaction, the organopolysiloxane preferably has a molecular weight notlarger than 1,500. Especially methylmethoxy polysiloxane is preferablebecause of ease of synthesis, good stability and high reactivity withthe surface of titanium dioxide.

The amount of the organopolysiloxane used is preferably in the range of0.1 to 3.0 parts by weight based on 100 parts by weight of titaniumdioxide. If the amount of the organopolysiloxane is smaller than 0.1part by weight, the effect of masking moisture is insufficient, and,cracks are liable to be formed on a film produced by a melt-extrusionlamination. The use of the organopolysiloxane in an amount exceeding 3.0parts by weight is not economically advantageous because the effect ofmasking moisture is approximately on the same level as that achievedwith the amount of 3.0 parts by weight, and further, the film producedhas rather deteriorated characteristics because of decomposition of theorganopolysiloxane.

The treatment of titanium dioxide with the organopolysiloxane is carriedout as follows. The organopolysiloxane is atomized, not onto the exposedsurface of a fluidized powder layer of titanium dioxide, but within thefluidized powder layer of titanium dioxide. When the organopolysiloxaneused is liquid, it is usually applied as it is. When theorganopolysiloxane applied is solid or viscous liquid, it is usuallyused as a solution in an organic solvent such as methanol or ethanol.The liquid organopolysiloxane or organopolysiloxane solution is atomizedas droplets having an average diameter of 1 to 100 μm. If the atomizeddroplets have an average diameter smaller than 1 μm, the droplets areliable to fly off. In contrast, if the atomized droplets have an averagediameter larger than 100 μm, the droplets are not uniformly dispersedand the resulting coating is not uniform.

The organopolysiloxane-applied titanium dioxide is preferably heated ata temperature of 100 to 150° C. for about on hour.

After the treatment with the organopolysiloxane, a metallic soap ismixed with the organopolysiloxane-treated titanium dioxide. The mixtureis kneaded together by using a kneader. The kneader may be conventionalbut a high-speed shearing kneader is preferable.

The metallic soap used is usually metallic salts of a higher fatty acid.As examples of the metallic soap, there can be mentioned zinc stearate,calcium stearate, barium stearate, magnesium stearate, aluminumstearate, calcium laurate, magnesium laurate and zinc laurate. Theamount of the metallic soap is preferably in the range of 1 to 5 partsby weight based on 100 parts by weight of titanium dioxide. If theamount of the metallic soap is too small, coated paper with a coatinghaving a good smoothness and containing no bubbles is difficult toobtain. If the amount of the metallic soap is too large, when athermoplastic resin composition is extrusion-laminated, a salient amountof smoke is discharged and the working environment is polluted, andfurther, the film characteristics of the resulting coating aredeteriorated.

To enhance the film characteristics of the coating formed on paper, thekneading of a mixture of the organopolysiloxane-treated titanium dioxidewith the metallic soap is preferably conducted within 24 hours aftercompletion of the treatment with the organopolysiloxane.

After the kneading of the metallic soap-incorporated mixture, themixture is then heat-treated at a temperature of 70 to 200° C. for 10minutes to 2 hours. If the heating temperature is lower than 70° C., acoating of a good state becomes difficult to obtain. In contrast, if theheating temperature is higher than 200° C., the organopolysiloxane isdeteriorated and the film characteristics of the coating become poor.

The heat-treatment of the kneaded mixture can be conducted either byheating the kneader having charged the mixture therein or by usinganother heating apparatus.

The titanium dioxide pigment which has been surface-treated in theabove-mentioned manner is then incorporated in a thermoplastic resin,and the mixture is kneaded together. The kneading can be conducted byusing a conventional kneader which includes, for example, a batchwisekneader (such as Banbury mixer), a twin-screw kneader and a single screwextruder having a kneading function.

The thermoplastic resin used is not particularly limited provided that acoating of the intended good state and characteristics is formedtherefrom. As examples of the thermoplastic resin, there can bementioned polyolefins such as polyethylene, polypropylene, anethylene-propylene copolymer (including random and block copolymers) andpolystyrene, as ABS resin, an AS resin, polyvinyl chloride, polyester,polyacetal, polycarbonate, poly-aromatic ethers, poly-aromatic estersand polysulfones, of these, polyolefins are preferable because a coatinghaving good film characteristics is obtained.

The surface-treated titanium dioxide pigment is incorporated in athermoplastic resin to prepare a thermo-plastic resin composition. Theamount of the surface-treated titanium dioxide pigment is usually in therange of 20 to 80% by weight based on the weight of the thermoplasticresin composition. When the thermoplastic resin composition hasincorporated therein a relatively small amount, i.e., 20 to 60% byweight, of the surface-treated titanium dioxide pigment, the resincomposition is used for forming a coating on paper or film. When thethermoplastic resin composition has incorporated therein a relativelylarge amount, i.e., 30 to 80% by weight, of the pigment, the resincomposition is used as a master batch for the preparation of athermo-plastic resin composition for forming a coating on paper or film.The master batch is usually in the form of a pellet, and is blended witha thermoplastic resin composition containing no pigment or containing avery minor amount of the pigment to form a thermoplastic resincomposition containing the pigment at a concentration desired forforming a coating on paper or film.

If the content of the surface-treated titanium dioxide pigment in themaster batch is smaller than 30% by weight, a thermoplastic resincomposition containing the pigment at the desired concentration isdifficult to prepare, and thus, the intended optical characteristicssuch as high opacity and high whiteness are difficult to achieve. Incontrast, if the content of the pigment in the master batch is largerthan 80% by weight, the pigment is difficult to uniformly disperse inthis master batch.

If the content of the surface-treated titanium dioxide in thethermoplastic resin composition for coating paper or film is smallerthan 20% by weight, the intended optical characteristics are difficultto obtain. In contrast, if the content of the pigment in the resincomposition is larger than 60% by weight, when a highly white coating orfilm is made therefrom, the coating or film has defects such as bubblescaused by volatile matter such as moisture accompanying the salientamount of the pigment.

The thermoplastic resin composition prepared by the above-mentionedprocess contains a desirably controlled amount, i.e., 10 to 1,400 ppm,of moisture. The moisture content is determined by a method wherein 3 gof the thermo-plastic resin composition is humidified in an atmospheremaintained at a temperature of 60° C. and a relative humidity of 60% for4 hours, exposed to a stream of air having an absolute humidity of 0.009kgH₂ O/kg of dry air at a temperature of 80° C. for 4 hours, and thenthe resin composition is deaerated at a temperature of 300° C. for 30minutes, and the moisture content is calculated by the followingequation:

    A=B/C

wherein A is moisture content, B is content (g) of moisture as measuredby the Karl Fischer method, and C is the content (g) of titanium dioxidepigment in the resin composition. In this specification, the moisturecontent is expressed in terms of the unit "ppm".

If the moisture content in the thermoplastic resin composition exceeds1,400 ppm, when a coating of film is made by extrusion lamination, thecoating or film has surface defects. In contrast, if the moisturecontent in the thermoplastic resin composition is smaller than 10 ppm,the surface-treated titanium dioxide pigment has a poor stability andthe coating or film has poor optical characteristics and surfacesmoothness. Preferably the content of moisture in the thermoplasticresin is in the range of 10 to 1,000 ppm.

When the moisture content in the thermoplastic resin composition iscontrolled in the above range, even though the content of thesurface-treated titanium dioxide pigment is increased, coated paper orfilm having the desired surface smoothness and good surface state can beobtained.

The invention will now be described by the following examples andcomparative examples that by no means limit the scope of of theinvention.

EXAMPLE 1

In 2,500 liters of pure water, 300 kg of commercially available rutiletitanium dioxide having an average particle diameter of 0.25 μm("Super-Titania G" supplied by Showa Titanium Co.) was dispersed. Anaqueous sodium aluminate solution was added to the aqueous dispersion inan amount such that the surface-treated titanium dioxide contained 0.5%by weight (expressed as alumina) of alumina hydrate. Then the dispersionwas diluted with dilute sulfuric acid, filtered, washed, and finallydried at 150° C. for 1 hour to give 270 kg of titanium dioxide havingsurface-treated with alumina hydrate.

By using a high-speed shearing mixer ("FM 75" supplied by Mitsui MiningCo.), 20 kg of the surface-treated titanium dioxide was stirred at arevolution of 700 rpm to form a good fluidized powder layer. Atwo-nozzle spray gun (Flat-spray 1/4J supplied by Spraying System Co.)was placed in the fluidized powder layer so that the entire nozzle waslost within the powder layer, while the spray gun was observed by thenaked eye. 200 g of methylmethoxy polysiloxane having a molecular weightof 250 to 500 was atomized through the spray gun by blowing a nitrogengas at a pressure of 5 kg/cm². Measurement of the average diameter ofthe atomized droplets as measured previously when atomized under thesame conditions revealed that the droplet diameter was 15 μm. Aftercompletion of atomizing, the surface treated titanium dioxide was driedat 150° C. for 1 hour, and then, 1 kg of zinc stearate was added andkneaded together with the titanium dioxide by a Henschel mixer.

20 kg of the thus-obtained titanium dioxide pigment powder was mixedwith 20 kg of low-density polyethylene ("J-REX LDL133K" supplied byJapan Polyolefins Co.). The mixture was melt-extruded through atwin-screw extruder whereby a master batch in the from of a pellethaving a diameter of about 3 mm and a length of about 5 mm was obtained.The content of moisture in the master batch as measured by theabove-mentioned method was 900 ppm.

The master batch was kneaded together with the above-mentionedlow-density polyethylene ("J-REX LDL133K") at a mixing ratio of 1/0.667by weight by using a labo-plastomill (an extruder with a screw having adiameter of 20 mm, supplied by Toyo Seiki K.K.) to give a polyethyleneresin composition. The content of moisture in this resin composition was910 ppm.

The polyethylene resin composition was melt-extruded by anextrusion-lamination procedure onto a surface of woodfree paper with athickness of 200 μm to form a laminate having a coating with a thicknessof 20 μm. The surface state of the coated paper was observed by thenaked eye to examine the degree of smoothness and the presence ofbubbles and micro-spots. The evaluation result of the surface state was"A", as expressed by the following four ratings.

A: the number of bubbles and micro-spots is smaller than 2 per 100 cm²and the coated surface is almost mirror-smooth.

B: the number of bubbles and micro-spots is at least 2 but smaller than5 per 100 cm² and the coated surface has a good smoothness.

C: the number of bubbles and micro-spots is at least 5 but smaller than10 per 100 cm² and the coated surface has somewhat poor smoothness.

D: the number of bubbles and micro-spots is at least 10 per 100 cm² andthe coated surface has poor smoothness.

EXAMPLE 2

By the same procedures as described in Example 1, a master batch and apolyethylene resin composition were made from the same rutile titaniumdioxide as used in Example 1 wherein the treatment of titanium dioxidewith alumina hydrate was not conducted and the average diameter ofatomized droplets was changed to 19 μm. All other conditions remainedthe substantially the same.

The production conditions are shown in Table 1. The evaluation resultsof the master batch, the polyethylene resin composition and the coatedpaper are shown in Table 2.

EXAMPLE 3

By the same procedures as described in Example 1, a master batch and apolyethylene resin composition were made from anatase titanium dioxidehaving an average particle diameter of 0.1 μm ("TCA123" supplied byTochem Products Co.) instead of rutile titanium dioxide wherein theaverage diameter of atomized droplets was changed to 30 μm. All otherconditions remained the substantially the same.

The production conditions are shown in Table 1. The evaluation resultsof the master batch, the polyethylene resin composition and the coatedpaper are shown in Table 2.

EXAMPLE 4

By the same procedures as described in Example 1, a master batch and apolyethylene resin composition were made from the same anatase titaniumdioxide as used in Example 3 wherein the treatment of titanium dioxidewith alumina hydrate was not conducted and the average diameter ofatomized droplets was changed to 60 μm. All other conditions remainedthe substantially the same.

The production conditions are shown in Table 1. The evaluation resultsof the master batch, the polyethylene resin composition and the coatedpaper are shown in Table 2.

EXAMPLES 5-20

Master batches and polyethylene resin compositions were made under thesame production conditions and by the same procedures as those recitedin Table 1. The other conditions and procedures which are not describedin Table 1 were the same as employed in Example 3.

In Examples 10, 11 and 12, methylhydrogen polysiloxane having amolecular weight of 200 to 400, dimethyl polysiloxane having a molecularweight of 150 to 300 and methylphenyl polysiloxane having a molecularweight of 300 to 800 were used, respectively, as the organopolysiloxane.

The evaluation results of the master batch, the polyethylene resincomposition and the coated paper are shown in Table 2.

COMPARATIVE EXAMPLES 1-9

Master batches and polyethylene resin compositions were made fromalumina hydrate-treated anatase titanium dioxide which was the same asthat used in Example 3. The amount of organopolysiloxane, the averagediameter of atomized droplets, the amount of metallic soap and theconditions under which heattreating was conducted after mixing werevaried so that at least one of these conditions was outside the claimedranges, as shown in Table 1.

The evaluation results of the master batch, the polyethylene resincomposition and the coated paper are shown in Table 2.

As seen from the data shown in Table 2, even though the content oftitanium dioxide pigment in the thermoplastic resin composition of thepresent invention is high, coated paper having a coating with goodsmooth surface and good surface state is obtained.

                                      TABLE 1                                     __________________________________________________________________________    Metallic                Content of                                            soap and Heat- Content of TiO.sub.2                                                                   moisture                                              amount*2 treating                                                                            in  in resin                                                                           in  in resin                                          (parts/  condition*3                                                                         master                                                                            composi-                                                                           master                                                                            composi-                                                                           State                                        Ex. No.                                                                           100 parts                                                                          Temp                                                                             Time                                                                             batch                                                                             tion batch                                                                             time of                                           *1  of TiO.sub.2)                                                                      (° C.)                                                                    (hr)                                                                             (wt. %)                                                                           (wt. %)                                                                            (ppm)                                                                             (ppm)                                                                              coating                                      __________________________________________________________________________     1  Zn-St                                                                            4 150                                                                              0.5                                                                              50  30     900                                                                               910                                                                              A                                             2  "  4 150                                                                              0.5                                                                              50  30     860                                                                               870                                                                              A                                             3  "  4 125                                                                              0.5                                                                              50  30   1,240                                                                             1,260                                                                              A                                             4  "  4 125                                                                              0..5                                                                             50  30     740                                                                               750                                                                              A                                             5  "  4 125                                                                              0.5                                                                              50  30   1,180                                                                             1,200                                                                              B                                             6  "  4 125                                                                              0.5                                                                              50  30     890                                                                               890                                                                              A                                             7  "  4 125                                                                              0.5                                                                              50  30   1,270                                                                             1,290                                                                              B                                             8  "  4 125                                                                              0.5                                                                              50  30     900                                                                               910                                                                              A                                             9  "  4 125                                                                              0.5                                                                              50  30   1,320                                                                             1,350                                                                              B                                            10  "  4 125                                                                              0.5                                                                              50  30   1,340                                                                             1,360                                                                              B                                            11  "  4 125                                                                              0.5                                                                              50  30   1,260                                                                             1,280                                                                              B                                            12  "  4 125                                                                              0.5                                                                              50  30   1,080                                                                             1,100                                                                              B                                            13  "  4 125                                                                              0.5                                                                              50  30   1,160                                                                             1,190                                                                              A                                            14  "  4.8                                                                             125                                                                              0.5                                                                              50  30   1,160                                                                             1,180                                                                              A                                            15  "  1.5                                                                             125                                                                              0.5                                                                              50  30   1,260                                                                             1,280                                                                              B                                            16  "  4 125                                                                              0.5                                                                              70  45     920                                                                               930                                                                              A                                            17  "  4 80 0.5                                                                              50  30   1,240                                                                             1,260                                                                              B                                            18  "  4 180                                                                              0.5                                                                              50  30     960                                                                               980                                                                              A                                            19  Ca-St                                                                            4 125                                                                              0.5                                                                              50  30   1,180                                                                             1,200                                                                              B                                            20  Zn-St                                                                            4 125                                                                              0.5                                                                              50  30   1,380                                                                             1,390                                                                              B                                            C1  "  4 125                                                                              0.5                                                                              50  30   1,940                                                                             1,990                                                                              D                                            C2  "  4 125                                                                              0.5                                                                              50  30   1,980                                                                             2,010                                                                              D                                            C3  "  4 125                                                                              0.5                                                                              50  30   1,780                                                                             1,820                                                                              D                                            C4  "  4 125                                                                              0.5                                                                              50  30   1,960                                                                             1,990                                                                              D                                            C5  "  4 125                                                                              0.5                                                                              50  30   1,620                                                                             1,650                                                                              C                                            C6  "  0.5                                                                             125                                                                              0.5                                                                              50  30   1,680                                                                             1,710                                                                              C                                            C7  "  6 125                                                                              0.5                                                                              50  30   1,680                                                                             1,720                                                                              C                                            C8  "  4  60                                                                              0.5                                                                              50  30   1,640                                                                             1,660                                                                              D                                            C9  "  4 220                                                                              0.5                                                                              50  30   1,580                                                                             1,610                                                                              C                                             C10                                                                              "  4 125                                                                              0.5                                                                              50  30   1,680                                                                             1,720                                                                              D                                            __________________________________________________________________________     *1 C: Comparative Example (outside the claim of invention)                    *2 Metallic soap                                                                ZnSt: Zinc stearate                                                           CaSt: Calcium stearate                                                      *3 Conditions (temperature and time) of heattreating the titanium dioxide     pigment mixture after the pigment mixture is kneaded with the metallic        soap.                                                                    

                                      TABLE 2                                     __________________________________________________________________________    Type of    Alumina                                                                            Organopoly-                                                                             Average                                             TiO.sub.2 and                                                                            hydrate                                                                            siloxane*3                                                                              diameter                                            average    treatment                                                                          and amount                                                                              of   Heat-treating                                  particle   and  (parts/   atomized                                                                           conditions*4                                   Ex. No.                                                                           diameter                                                                             amount                                                                             100 parts droplets                                                                           Temp                                                                             Time                                        *1  (μm)                                                                              *2 (wt %)                                                                          TiO.sub.2)                                                                              (μm)                                                                            (° C.)                                                                    (hr)                                        __________________________________________________________________________     1  Rutile                                                                            0.25                                                                             0.5  MeMeO-PS                                                                            1.0 15   150                                                                              1                                            2  Rutile                                                                            0.25                                                                             NC   MeMeO-PS                                                                            1.0 19   150                                                                              1                                            3  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 30   150                                                                              1                                            4  Anatase                                                                           0.2                                                                              NC   MeMeO-PS                                                                            1.0 60   150                                                                              1                                            5  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            0.2 21   150                                                                              1                                            6  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.7 17   150                                                                              1                                            7  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            2.8 16   150                                                                              1                                            8  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0  3   150                                                                              1                                            9  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 90   150                                                                              1                                           10  Anatase                                                                           0.2                                                                              0.5  MeH-PS                                                                              1.0 18   150                                                                              1                                           11  Anatase                                                                           0.2                                                                              0.5  DMe-PS                                                                              1.0 19   150                                                                              1                                           12  Anatase                                                                           0.2                                                                              0.5  MePh-PS                                                                             1.0 20   150                                                                              1                                           13  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 22   150                                                                              1                                           14  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 16   150                                                                              1                                           15  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 18   150                                                                              1                                           16  Anatase                                                                           0.2                                                                              0.5  MeMeo-PS                                                                            1.0 20   150                                                                              1                                           17  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 13   150                                                                              1                                           18  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 17   150                                                                              1                                           19  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 24   150                                                                              1                                           20  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 23   150                                                                              1                                           C1  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 0.8  150                                                                              1                                           C2  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 0.5  150                                                                              1                                           C3  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 120  150                                                                              1                                           C4  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            0.06                                                                              23   150                                                                              1                                           C5  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            3.5 19   150                                                                              1                                           C6  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            0.9 18   150                                                                              1                                           C7  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.2 24   150                                                                              1                                           C8  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            0.8 11   150                                                                              1                                           C9  Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 18   150                                                                              1                                            C10                                                                              Anatase                                                                           0.2                                                                              0.5  MeMeO-PS                                                                            1.0 25   150                                                                              1                                           __________________________________________________________________________     *1 C: Comparative Example (outside the claim of invention)                    *2 Aluminum hydrate treatment                                                   NC: not conducted                                                             0.5: amount of alumina hydrate used is 0.5 wt. % as alumina                 *3 Organopolysiloxane                                                           MeMeOPS Methylmethoxypolysiloxane*                                            MeHPS Methylhydrogenpolysiloxane                                              DMePS Dimethylpolysiloxane                                                    MePhPS Methylphenylpolysiloxane                                             *4 Conditions (temperature and time) of heattreating the                      organopolysiloxaneatomized titanium dioxide                              

What is claimed is:
 1. A process for preparing a thermoplastic resincomposition comprising a thermoplastic resin and a titanium dioxidepigment, which comprises the steps of:atomizing 0.1 to 3.0 parts byweight of an organopolysiloxane as droplets having an average diameterof 1 to 100 μm within a fluidized powder layer of 100 parts by weight ofa titanium dioxide pigment; mixing 1 to 5 parts by weight of a metallicsoap with 100 parts by weight of the organopolysiloxane-treated titaniumdioxide pigment; heat-treating the thus-obtained mixture at atemperature of 70 to 200° C. for a period of 10 minutes to 2 hours; andthen mixing a thermoplastic resin with the heat-treated mixture.
 2. Theprocess for preparing a thermoplastic resin composition as claimed inclaim 1, wherein the titanium dioxide pigment has an average particlediameter of 0.1 to 0.3 μm.
 3. The process for preparing a thermoplasticresin composition as claimed in claim 1, wherein the titanium dioxidepigment has been surface-treated with aluminum oxide hydrate in anamount of 0.1 to 0.8 parts by weight as aluminum oxide based on 100parts by weight of the titanium dioxide pigment.
 4. The process forpreparing a thermoplastic resin composition as claimed in claim 1,wherein the organopolysiloxane is a compound with a siloxane bond,represented by the following formula: ##STR2## wherein R₁, R₂, R₃, R₄,R₅ and R₆ independently represent a hydrogen atom, a hydroxyl group, analkyl group having 1 to 7 carbon atoms, a cycloalkyl group having 3 to 8carbon atoms, an aryl group having 6 to 14 carbon atoms, an aralkylgroup having 7 to 12 carbon atoms, an alkenyl group having 2 to 8 carbonatoms, an alkoxy group having 1 to 7 carbon atoms, an aryloxy grouphaving 6 to 14 carbon atoms, an aralkyloxy group having 7 to 12 carbonatoms, an alkenyloxy group having 2 to 8 carbon atoms, and n is aninteger of at least
 1. 5. The process for preparing a thermoplasticresin composition as claimed in claim 1, wherein theorganopolysiloxane-atomized titanium dioxide pigment is heat-treated ata temperature of 100 to 150° C. before the mixing with the metallicsoap.
 6. The process for preparing a thermoplastic resin composition asclaimed in claim 1, wherein 20 to 80% by weight, based on the weight ofthe thermoplastic resin composition, of the thermoplastic resin is mixedwith the heat-treated titanium dioxide pigment mixture.